The present invention relates generally to devices used to determine the presence of an object or materials near a sensor and, more particularly, to proximity sensors capable of sensing the presence of materials that are conventionally difficult to detect.
A device or system having the ability to detect the presence, level, or quantity of particular materials, commonly referred to as a proximity detector, has many uses. For example, proximity detectors may be used to detect or sense the level of grain, aggregate, fluids or other materials in a storage container, or to detect the presence of a metal part on a production line. While there are different types of proximity sensors available, each suffers from certain disadvantages, making the sensors less desirable for particular applications.
One type of proximity detector is a conventional capacitive sensor. As is known, conventional capacitive sensors are sensitive to changes in the dielectric constant of materials. These sensors typically include the material to be sensed as part of the dielectric material of a tuning capacitator. As the material to be sensed comes into spatial proximity of the capacitive sensor, the dielectric constant of the tuning capacitator changes, altering the capacitance of the tuning capacitator. The altered capacitance either changes the oscillation frequency of the tuned system or the time constant of an RC delay circuit. Either the oscillation frequency or the time constant is then compared to a nominal value (i.e., when the material to be sensed is not near the sensor) to determine the presence of the material. While conventional capacitive sensors are generally useful for detecting certain materials based on the capacitance of the materials, these sensors are generally unable to detect materials based on other electrical properties of the material, such as resistance and/or inductance. Additionally, conventional capacitive sensors are susceptible to changes in certain environmental conditions such as temperature variations.
Ultrasonic proximity detectors exploit reflections of sound waves off an object to detect the presence of the object. The detectors transmit a sound pulse and detect a returning sound wave reflected off the target. By measuring the elapsed round-trip time of the sound wave, the distance to the object can be determined. While these systems may be satisfactory for determining the presence of an object where the distance between the sensor and the object is at least several inches, they are not generally well suited to detect objects that may be very close to the sensor (i.e., less than about two inches) because the echo (i.e., the returning sound wave) becomes difficult to distinguish from the initial transmitted sound pulse. Additionally, the ability of an ultrasonic proximity detector to accurately detect an object or materials may be dependent upon the shape, configuration or surface of the object or materials because, in the typical configuration wherein the sensor acts both as the ultrasonic transmitter and receiver, the transmitted sound wave must reflect off the object or materials and toward the ultrasonic transducer.
Another conventional type of proximity detector is an electromagnetic (EM) wave-based system. EM proximity detectors are similar to ultrasonic systems, but use electromagnetic waves (e.g., microwaves, lasers, and the like) rather than sonic waves. Because the timing requirements for these EM detectors are demanding (typically picosecond resolution), EM-based detectors are expensive. Additionally, EM sensors suffer from at least the same primary disadvantage as ultrasound sensors, i.e., the EM sensors have difficulty detecting objects that may be in close proximity to the sensor because the sensors have difficulty separating the initial transmitted pulse from the returned pulse unless there is a sufficient spacing between the sensor and the object to be detected.
An improved low-power proximity detector is a tuned sensor element. Using this technique, a monopole or dipole element is used as the sensor (i.e., antenna), which is coupled to circuitry designed to be sensitive to the Q of the tuned circuit. As the sensor comes near a lossy material (i.e., a material having a complex permittivity or complex permeability), the Q of the tuned circuit decreases. This decrease in Q can be detected to determine if the material is in proximity to the sensor. Tuned sensors may be packaged as part of an integrated circuit such as, for example, model CS-209A, manufactured by Cherry Semiconductor Corp. of East Greenwich, RI. Conventional tuned sensors, however, are typically designed and/or tuned to detect either high permittivity or high conductivity materials by having a fixed threshold on their detection comparator.
Another type of proximity detector having an oscillation circuit is disclosed in U.S. Pat. No. 5,832,772. This circuit includes a bipolar transistor having its collector coupled to a straight monopole antenna having a characteristic impedance that is dependent upon materials in proximity to the antenna. The transistor and antenna form a resonant circuit that oscillates at a nominal frequency with an amplitude that is dependent on the presence of certain materials in proximity to the antenna. The base of the transistor is coupled to a bias potential, and the emitter of the transistor is coupled to a control circuit which turns the oscillator on and off. A detector circuit is coupled to the emitter junction of the transistor to detect changes in the amplitude of the oscillations as a result of the proximity of the antenna to certain materials. While this proximity detector circuit may have uses in certain areas, it has been found that for particular applications, such as, for example, sensing low permittivity materials, the circuit has certain disadvantages including a lack of sensitivity and susceptibility to fluctuations in external temperature and variations in the components used in the circuit.
Therefore, the need remains for a proximity detector having an oscillation circuit that is more sensitive to low permittivity materials. Preferably, the proximity detector would also be less susceptible to environmental changes (e.g., temperature) and component manufacturing tolerances (e.g., transistor gain) and would be able to detect the presence of conventionally hard-to-detect materials.
An apparatus for detecting the proximity of conventionally hard-to-detect materials having these features and satisfying these needs has now been developed. The current invention allows for accurate detection of the proximity of hard-to-detect materials. As used herein, the phrase hard-to-detect materials generally means materials having a low relative permittivity (e.g., epsilon approximately 2) such as, for example, bulk materials having a relatively low water content (e.g., animal feeds), petroleum products, and polymers (i.e., plastics). Of course, the present invention may also be used to detect other materials or objects without departing from the spirit and scope of the invention. For example, the present invention may also be used to detect the presence of certain high permittivity materials such as, for example, certain aqueous solutions, antifreeze (ethylene glycol), and/or other high conductivity materials, such as metals. The present invention also allows for the detection of these materials using a novel circuit design that operates on low power, is capable of detection across a wide temperature range, and is less sensitive to component variations.
Broadly speaking, the proximity detector of the present invention operates by reacting to certain electrical properties of materials near or surrounding the detector""s sensor. The preferred proximity detector includes a high-frequency oscillator that is electrically coupled to a tuned circuit that includes a sensor element. As the sensor element comes in near proximity of a lossy material (i.e. a material having a complex permittivity or complex permeability), the Q of the tuned circuit decreases, thereby diminishing the amplitude of the oscillations of the high-frequency oscillator. By periodically detecting and measuring the amplitide of the oscillations, the proximity detector can determine the presence of a sensed material near the sensor element.
Preferably, the proximity detector of the present invention includes an oscillator for producing an output signal at a predetermined frequency. Preferably, the oscillator generates its output signal using a conventional transistor having a base, an emitter and a collector, wherein the base-emitter junction rectifies resonant signals at the collector to produce the output signal. Preferably, the oscillator further includes negative feedback for reducing the sensitivity of the closed loop gain to variations in the transistor""s open loop gain and for stabilizing the frequency of the oscillator. The negative feedback is preferably provided by a resistive element operatively coupled to the emitter and a capacitive element operatively coupling the resistive element to the collector.
The preferred proximity detection system also includes a sensor operatively coupled to the oscillator for varying the level of the oscillator output signal in response to variations in external electrical properties near the sensor and a controller operatively coupled to the oscillator to receive the oscillator output signal and being responsive to variations of predetermined magnitudes of the amplitudes of the oscillator output signal for detecting the presence or level of the material. Preferably, the sensor has a novel geometry consisting of two substantially flat plates that may be sized and positioned to control the polarization orientation of the electric field lines extending from the sensor so that the field lines extend substantially normal to the surface of the plates and, therefore, extend farther away from the sensor and potentially intersect a greater volume of material. In one embodiment, the two plates are positioned perpendicular to each other. In another embodiment, the two plates are co-planar with a lateral separation region therebetween. Using either configuration, a high dielectric material (e.g., alumina) may be placed on the side of the sensor proximal to the material to be detected to further reposition the electric field lines in a direction toward the material to be sensed.
In a preferred embodiment, the controller issues a gate signal that is coupled to the oscillator for turning the oscillator on at a predetermined frequency and for a predetermined pulse width. Synchronously with the control of this gate signal, the controller receives the oscillator output signal, thus reducing the overall power requirements of the system.